A STUDY OF VERY SMALL DEFECT CLUSTERS IN IRRADIATED MAGNESIUM OXIDE USING LONG-WAVELENGTH NEUTRON SCATTERING MEASUREMENTS

The scattering of 5.4 neutrons by defects in single crystals of neutronirradiated magnesium oxide has been measured over the angular range 5-90°. The effects of irradiation dose and subsequent annealing treatments have been studied in the range 5×1019-2·5×10 21 neutrons cm-2 (>1 MeV) and 350-1460°c respectively. The shapes of the curves of the observed cross section against angle were found to be sensitive to the annealing temperature but were influenced only slightly by the irradiation dose. In all cases the cross section decreases with angle, this decrease becoming more pronounced the higher the annealing temperature. Evidence is presented which suggests that the defect scattering occurs predominantly from vacancies. The results were compared with the scattering expected from a number of vacancy aggregate models and correlations were obtained for aggregates containing between 1 and about 100 vacancies per cluster in the temperature range 150°c (during irradiation) to 1460°c respectively. At the two extremities of this size range it is shown that the present conclusions are in reasonable agreement with the results obtained by other workers from electron spin resonance and electron microscope observations respectively. After a 4 MeV electron irradiation at 500°c to a dose of 1·4×1019 electrons cm-2 no defect scattering could be observed within the accuracy of the cross-section measurements, ±0·5 mbn sr-1 molecule-1.